The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines employ ink-jet technology for producing printed media. Hewlett-Packard's contributions to this technology are described, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992), and Vol. 45, No.1 (February 1944).
Generally, an ink-jet image is formed when a precise pattern of dots is ejected from a drop generating device known as a "print head" onto a printing medium. The typical ink-jet print head has an array of precisely formed nozzles attached to a thermal ink-jet print head substrate. The substrate incorporates an array of firing chambers that receive liquid ink (colorant dissolved or dispersed in a solvent) from an ink reservoir. Each chamber has a thin-film resistor, known as a "firing resistor", located opposite each nozzle so ink can collect between the firing resistor and the nozzle. When electric printing pulses heat the thermal ink-jet firing resistor, a small volume of ink adjacent the firing resistor is heated, vaporizing a bubble of ink, and thereby ejecting a drop of ink from the print head. The droplets strike the printing medium and then dry to form "dots" that, when viewed together, form the printed image.
In general, the ink within a print head has an operating pressure chosen between two limiting conditions. The operating pressure must be sufficiently low so that during operation the ink remains within the print head and does not run freely out of the firing chambers and the nozzles. On the other hand, the operating pressure of the print head must be sufficiently high so when the firing resistor is heated, the vaporized bubble of ink can over come the pressure and eject a droplet of ink from the firing chamber. Most print heads today operate at a slight vacuum, typically in a pressure range of between about -2" (minus two inches) of water to about -30" (minus thirty inches) of water.
In the future ink-jet print heads may be located in printers having pressurized ink supplies. A pressurized ink supply enables ink to be supplied to the print head at higher flow rates than non-pressurized systems. The ink in such a system is pressurized in a reservoir to a supply pressure of between about -5" (minus five inches) of water to about +80" (plus eighty inches) of water and is delivered to the print head using either a tube or a conduit. A back pressure regulator is normally located near the print head to reduce the supply pressure of the ink down to the operating pressure of the print head.
Needless to say, such pressurized ink supply systems that operate above atmospheric pressure have the potential for developing leaks that may result in ink freely running out of the print head or its housing. Thus, there is a need for a fail/safe valve to prevent such incidents.
It should be appreciated that there was no need to solve this problem in previous ink delivery systems because such systems operated on capillary pressure and at a slight vacuum. Further, these unpressurized systems did not incorporate a back pressure regulator that could fail to shut during operation and that could over pressurize the print head to the point of spraying ink.